The present invention relates to an image forming apparatus such as an electrophotographic copying machine. More specifically, the invention relates to an image forming apparatus for transferring a toner image, which is obtained by developing a latent image on an image carrying member, onto a recording paper by physical means using a transfer belt unit having a transfer belt which rotates, for conveying the recording paper with the toner image to fixing means, and for fixing the toner image by the fixing means.
Many methods and apparatuses for obtaining multi-color images by the electrophotographic method have been proposed. As one of them, a method for repeating the formation of latent images according to the number of decomposed colors of an original image on a photoconductor which is an image carrying member and developing them with color toners; for superimposing the multi-color toner images on the photoconductor; and for transferring them onto a transfer material so as to obtain a multi-color image has been proposed and executed. The basic process of multi-color image forming is mentioned in Japanese Patent Laid-Open Nos. 60-75850, 60-76766, 60-95456, 60-95458, and 60-158475 by the applicant of this specification.
In an image forming apparatus such as an electrophotographic copying machine, the peripheral surface of, for example, a photosensitive drum which is an image carrying member is uniformly charged by a charging unit and an image is exposed on the peripheral surface of the photosensitive drum so as to form an electrostatic latent image. This latent image is developed and visualized to a toner image by developing means. This toner image formed on the peripheral surface of the photosensitive drum is transferred onto a recording paper which is fed in exact timing by physical means, and the recording paper whereon the toner image is transferred is separated from the photosensitive drum and conveyed to fixing means. Further the toner image is fixed on the recording paper, and then the recording paper is ejected from the image forming apparatus.
As to transferring of the toner image, which is formed on the peripheral surface of the photosensitive drum, onto the recording paper, the recording paper is reversely charged to the charging of the toner by a transfer unit which discharges from the back of the recording paper and transferred onto the recording paper. After transferring, the recording paper is discharged by a high AC voltage applied by a separation unit and separated from the photosensitive drum. However, it is difficult to surely maintain the transfer property and separation property. Particularly, when the diameter of the photosensitive drum is large, the recording paper cannot be separated easily; that is, after the toner image is transferred, the recording paper is kept adhered to the photosensitive drum without being separated, causing jamming. A transfer belt unit is used as an art for improving this disadvantage.
The transfer belt unit rotates a transfer belt which is stretched by a plurality of holding rollers at the same speed as that of the photosensitive drum. The above transfer belt unit has charging means for supplying an electric charge to the recording paper so as to adhere to the transfer belt by electrostatic force. The recording paper which is adhered to the transfer belt comes in contact with the photosensitive drum in the transfer section and a high voltage which is reverse to the charging of the toner is applied to the recording paper in the transfer section under constant current control so as to transfer the toner image. In such a transfer belt unit, a high transfer efficiency and separation effect can be obtained in the transfer section.
Such a transfer belt unit can be used as a particularly desirable unit in a color image forming apparatus for superimposing toner images on the photosensitive drum and for transferring them onto the recording paper at a time. In such a color image forming apparatus, a plurality of developing units are mounted at the periphery of the photosensitive drum so as to superimpose toner images on a photoconductor, so that the drum diameter increases, and a sufficient separation performance cannot be obtained by the conventional electrostatic transfer and separation method, and a sure separation performance is necessary. Furthermore, the method for superimposing toner images requires a large amount of adhering toner, so that a large transfer charge is necessary and a large transfer charge holding capacity is necessary. The transfer belt unit is superior in the above two points.
When the aforementioned image forming apparatus is installed in a humid place or used in a humid season, the resistance of a recording paper is changed because the recording paper absorbs moisture and the water content increases, and toner images formed on the photosensitive drum are transferred defectively, and the image quality lowers. When the humidity is low, the transfer current becomes insufficient and toner images are transferred defectively.
To solve the above problems, a heater is used conventionally to heat a recording paper. However, it is impossible to finely adjust the humidity by heating according to the percentage of moisture content of the recording paper. Furthermore, a method that a temperature and humidity meter is mounted in an image forming apparatus so as to detect changes in the environment and to set the transfer conditions or a method that measuring means for measuring the resistance of a recording paper is mounted so as to set the transfer conditions from the measured resistance is conventionally used. However, new installation of such means causes a complicated structure and an increase in cost.
The object of the present invention is to provide an image forming apparatus having a transfer belt unit for allowing good transfer regardless of the temperature and humidity or the paper type without using such a complicated means.